Proof-of-concept Rust wrapper

This contains Rust wrappers around enough of the Python API to implement Rust versions of the `battery_client` and `run_scanner` examples. The goal is to gather feedback on the approach, and of course to show that it is possible.

The module structure mirrors that of the Python. The Rust API is not optimally Rust-y, but given the constraints of everything having to delegate to Python, it's at least usable.

Notably, this does not yet solve the packaging problem: users must have an appropriate virtualenv, libpython, etc. [PyOxidizer](https://github.com/indygreg/PyOxidizer) may be a viable path there.

rust/src/wrapper/mod.rs

@@ -0,0 +1,94 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Types that wrap the Python API.
+//!
+//! Because mutability, aliasing, etc is all hidden behind Python, the normal Rust rules about
+//! only one mutable reference to one piece of memory, etc, may not hold since using `&mut self`
+//! instead of `&self` is only guided by inspection of the Python source, not the compiler.
+//!
+//! The modules are generally structured to mirror the Python equivalents.
+
+// Re-exported to make it easy for users to depend on the same `PyObject`, etc
+pub use pyo3;
+use pyo3::{
+    prelude::*,
+    types::{PyDict, PyTuple},
+};
+pub use pyo3_asyncio;
+
+pub mod adv;
+pub mod assigned_numbers;
+pub mod core;
+pub mod device;
+pub mod gatt;
+pub mod gatt_client;
+pub mod hci;
+pub mod logging;
+pub mod profile;
+pub mod transport;
+
+/// Convenience extensions to [PyObject]
+pub trait PyObjectExt {
+    /// Get a GIL-bound reference
+    fn gil_ref<'py>(&'py self, py: Python<'py>) -> &'py PyAny;
+
+    /// Extract any [FromPyObject] implementation from this value
+    fn extract_with_gil<T>(&self) -> PyResult<T>
+    where
+        T: for<'a> FromPyObject<'a>,
+    {
+        Python::with_gil(|py| self.gil_ref(py).extract::<T>())
+    }
+}
+
+impl PyObjectExt for PyObject {
+    fn gil_ref<'py>(&'py self, py: Python<'py>) -> &'py PyAny {
+        self.as_ref(py)
+    }
+}
+
+/// Wrapper to make Rust closures ([Fn] implementations) callable from Python.
+///
+/// The Python callable form returns a Python `None`.
+#[pyclass(name = "SubscribeCallback")]
+pub(crate) struct ClosureCallback {
+    // can't use generics in a pyclass, so have to box
+    #[allow(clippy::type_complexity)]
+    callback: Box<dyn Fn(Python, &PyTuple, Option<&PyDict>) -> PyResult<()> + Send + 'static>,
+}
+
+impl ClosureCallback {
+    /// Create a new callback around the provided closure
+    pub fn new(
+        callback: impl Fn(Python, &PyTuple, Option<&PyDict>) -> PyResult<()> + Send + 'static,
+    ) -> Self {
+        Self {
+            callback: Box::new(callback),
+        }
+    }
+}
+
+#[pymethods]
+impl ClosureCallback {
+    #[pyo3(signature = (*args, **kwargs))]
+    fn __call__(
+        &self,
+        py: Python<'_>,
+        args: &PyTuple,
+        kwargs: Option<&PyDict>,
+    ) -> PyResult<Py<PyAny>> {
+        (self.callback)(py, args, kwargs).map(|_| py.None())
+    }
+}